This invention relates to thermal ink jet printing systems and, more particularly, to an improved printhead design incorporating several levels of interconnection for the resistive thermal energy generators.
Thermal ink jet printers are well known in the prior art as exemplified by U.S. Pat. Nos. 4,463,359 and 4,601,777. In the systems disclosed in these patents, a thermal printhead comprises one or more ink-filled channels communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A plurality of resistors are located in the channels at a predetermined distance from the nozzle. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper. In typical applications, ink droplets can be ejected at a rate of 5 kHz, giving rise to process speeds of up to 15 inches per second at 300 spots per inch printing resolution. To achieve practical print speeds, it is necessary to print with arrays of .apprxeq.20 or more nozzles which are constructed preferably, at the same ptich as pixels to be printed. Printers with small nozzle count use a scanning printhead and typically have print speeds of .apprxeq.1 page per minute (ppm). In order to print at speeds above .apprxeq.10 ppm, it is necessary to build a pagewidth print bar which typically contains several thousand jets. With process speeds of 15 inches per second, it is possible to print over 100 ppm with such architectures at 300 spi resolution. Therefore, to enable high through put thermal ink jet print engines, pagewidth print bars are essential.
The printhead design for the prior art systems described above place the thermal energy generators (resistors) on at least one wall of a small diameter capillary tube which contains the ink. The performance of the transducer depends strongly on the distance between the resistor and the nozzle. Drop size, drop velocity, and frequency of ink droplet ejection all depend on the distance between the resistor and the nozzle. Three hundred spi printing performance is optimized when the resistor begins about 120 .mu.m behind the nozzle. The proximity of the resistors to the nozzle, coupled with the high packing density necessary for high density printing have the implication that electrical front lead connection to one end of the resistors must be made across the front of the resistor array. The short distance from the nozzle to the resistor requires the front lead to be narrower than 120 .mu.m. For arrays of jets designed to operate up to a couple of ppm, the configuration where one end of the resistors is connected in common from both ends of the array is satisfactory. The problems with wider arrays, such as pagewidth, emerge because of the resistor energy requirement for printing, coupled with higher common lead resistance.
As mentioned previously, the thermal ink jet process uses rapid boiling of ink for drop ejection. Electrical heating pulses are applied for a few microseconds and must dissipate sufficient energy in the resister to raise it's surface temperature to about 300.degree. C. in order for bubble nucleation to occur. Typical energies required for drop ejection are between 10 and 50 microjoules (.mu.j), depending on the transducer structure and design. It is necessary to apply the energy within a short time, such as 5 .mu.sec. Therefore, about 8 watts are being dissipated during the heating pulse. The current necessary for heating depends on the resistance value of the transducer. If a resistance value of 200 .OMEGA. is chosen, then 200 mA of current is required and the device operates at 40 V. It is desirable to use high operating voltages so that currents are lowered, but high voltage adversely effects resistor lifetime. Therefore, a moderate voltage such as 40 or 60 V is chosen.
Another requirement of the circuit used for thermal ink jet printing is imposed by the drop ejection frequency (.apprxeq.5 kHz or 200 .mu.sec) and the heating pulse length of .apprxeq.5 .mu.sec. Only 40 jets can be fired over the 200 .mu.sec time. Currently yield and process technology allow monolithic integration of up to .apprxeq.200 jets with good yield. Therefore, 4 or 5 jets must be simultaneously fired. The exact number fired during any particular time depends on the document data being printed. In order for the threshold for drop ejection to be the same when one jet or all jets are fired, the lead which connects the resistors to the power supply must have negligible resistance in comparison with the resistive elements. For the case just discussed, 4 simultaneously fired jets have a total resistance of 50 .OMEGA.. Two hundred jets at 300 spi is 0.666 inches, or 17,000 .mu.m. The width of the metallization in front of the resistors is .apprxeq.100 .mu.m, so there is 170 .quadrature. of metal. For typical commercial metal thickness (1.25 .mu.m) and deposition techniques, aluminum has a sheet resistance of 0.032 .OMEGA./.quadrature.. Therefore, the common metal lead has an end to end resistance of 5.5 .OMEGA.. By connecting the metal on both ends, the resistance seen by the middle 4 resistors is 1.35 .OMEGA., or 2.7% of the resistor resistance. From this example, it can be seen that as the number of jets within a module grows, more jets must be simultaneously fired and the parasitic resistance effect caused by the aluminum common connection increases. The practical upper limit before an alternative approach needs to be considered is a consequence of the overvoltage which will be applied when only one resistor element is fired, given that all elements need to fire if selected. Overvoltage increases power dissipation, shortens element lifetime, and causes drop nonuniformity. For the devices considered here, 4 to 6 simultaneously fired jets is the maximum which is practical.
In addition to the problem of the parasitic resistance effect, a second problem when using the aluminum common connection for wide arrays is the connection of the common between a plurality of chips which have been butted together to form the wide array. In order to butt together arrays of modules, each module must terminate so the spacing between it and its neighbors does not give rise to a noticeable and undesirable stitch error. It is well known that printing irregularities as small as 25 .mu.m can be seen. Therefore, the modules must be within a few microns of their correct location. As an example, at 300 spi, 84.5 .mu.m is the pixel spacing. The thermal ink jet channel structure takes up about 65 um, leaving .apprxeq.20 .mu.m for creation of a butted joint. The 20 .mu.m joint can not deviate more than .+-.5 .mu.m before perceptible image quality degradation occurs. There is insufficient space at the ends of the module to make a low resistance connection to the common power lead which runs along the front edge of the module. Even when single modules containing many resistors are fabricated and front common leads can be brought out at the ends of the array, it may be desirable to make additional interconnections to the common in order to avoid parasitic voltage drop when many elements are simultaneously fired.
According to the present invention, the common connection utilized in the prior art is modified by forming two commons and interconnecting them. By providing a second common, the first common located between the resistor and nozzle can be made relatively narrow enabling the resistor to be located at an optimum distance upstream of the nozzle without being restricted by the width of the unmodified wider common. The resistors are connected to the heating pulse source by a low resistance structure which crosses over, or under, the second common. In one embodiment the low-resistance cross-over structure is a heavily-doped polysilicon layer and the second common is aluminum. Other possible combinations include an n+diffusion in a p type wafer and aluminum; refactory metal silicides and aluminum. These embodiments have the effect of decreasing the parasitic resistance associated with the single common and provide additional space to make the interconnection between butted-together chips. More particularly, the invention is directed towards an ink jet printhead of the type having a plurality of channels, each channel being supplied with ink and having an opening which serves as an ink droplet ejecting nozzle a heating element being positioned in each channel, ink droplets being ejected from the nozzles by the selective application of current pulses to the heating elements in response to data signals from a data signal source, the heating elements transferring thermal energy to the ink causing the formation and collapse of temporary vapor bubbles that expel the ink droplets, said printhead further comprising a first and second electrically conductive common return, said common returns interconnected by leads extending between said heating elements, said heating elements connected between said first common return and said data signal source by a low resistance connection which is formed beneath or above said second common return.